The present invention relates to the manufacture of integrated systems, and more particularly to a method of forming optical cores. Optical cores manufactured according to the present invention are useful in a variety of applications, and are particularly useful in integrated optical systems.
Integrated optical systems are an emerging technology that offer solutions to some previously unsolvable technological problems. Thus, integrated optical systems are becoming an increasingly important technology. Generally, optical systems utilize pulses of light rather than electric current to carry out such functions as data transmission, data routing, or other forms of data communication or data processing. One important structure commonly utilized in optical systems is an optical waveguide.
Optical waveguides are used to confine and direct light between the various components of an optical system. For example, optical waveguides may be used to carry Dense Wavelength Division Multiplexed (DWDM) light, which is used to increase the number of wavelengths in a single waveguide to achieve a higher aggregate bandwidth. FIG. 1 is a cross-sectional view of an optical fiber waveguide. The general structure of an optical waveguide 100 comprises two principal components: a core 103 and a cladding layer 102. The core 103 is the inner part of the fiber through which light is guided. It is surrounded by the cladding layer 102, which generally has a lower refractive index so that a light ray 105 in the core 103 that strikes the core/cladding boundary at a glancing angle is confined within the core 103 by total internal reflection. The confinement angle θc represents an upper limit for the angle at which the light ray 105 can strike the boundary and be confined within the core 103.
As more signals are incorporated into an optical waveguide channel, there is an increasing demand for components to route, switch, drop, and guide these light signals to their final destination. Many photonic components make this technology possible. These components include filters, modulators, amplifiers, couplers, multiplexers, optical cross-connects, Arrayed Waveguide Gratings (AWG), power splitters, and star couplers to name only a few. However, as optical technology matures, it is desirable to monolithically integrate various photonic components onto a single structure such as, for example, a silicon or glass substrate. Generally, processes for manufacturing an optical core on a substrate must result in structures that have good optical qualities. Additionally, such manufacturing processes must result in optical cores that are highly uniform across the substrate.
Attempts to integrate optical waveguides and photonic components onto a single chip have faced many challenges. For example, one optical waveguide core characteristic that is important to the performance of an optical system is propagation loss, which is also referred to as attenuation. Attenuation refers to the loss of light energy as a pulse of light propagates down a waveguide channel. The two primary mechanisms of propagation loss are absorption and scattering. Absorption is caused by the interaction of the propagating light with impurities in the waveguide channel. For example, electrons in the impurities may absorb the light energy and undergo transitions. The electrons may then give up the absorbed energy by emitting light at other wavelengths or in the form of vibrational energy (i.e., heat or phonons). Thus, the manufacturing processes used for building the optical waveguide cores must minimize the introduction of impurities that lead to propagation loss from absorption.
The second primary mechanism, scattering, results from geometric imperfections in the fiber that cause light to be redirected out of the fiber, thus leading to an additional loss of light energy. Accordingly, optical waveguide core uniformity is of paramount importance. Therefore, for satisfactory optical waveguides, the manufacturing process has the additional critical requirement of providing integrated optical waveguide core structures that are highly uniform across the substrate structure upon which they are built. Furthermore, the manufacturing process must maintain such uniformity across multiple process runs.
Another important parameter relating to optical cores is stress, which in one form is related to the difference in the coefficient of thermal expansion between the optical core and surrounding structures. Stress can produce a density change in the optical core structure. Typically, stress in an optical core is anisotropic. Anisotropic stress causes the permittivity and refractive index of core to be functions of direction, which causes the light to deleteriously propagate faster in one direction than in other directions. For example, the refractive index along the direction of the stress can be different from the refractive index perpendicular to it, causing light polarized along the direction of the stress to propagate at a different rate than light polarized perpendicular to the direction of the stress. This phenomena is sometimes referred to as birefringence, and can lead to distortion of the optical signals in a fiber. Problems associated with stress become compounded as optical cores are manufactured on larger wafer sizes. In particular, stress on larger wafers (e.g., 200 mm and up) can lead to bowing, which interferes with subsequent lithography during patterning of the core.
Another parameter critical to manufacturing integrated optical cores is refractive index uniformity. Uniformity in optical waveguide applications requires an extremely tight film homogeneity across the substrate and between wafer to wafer. Prior art integrated circuit manufacturing processes have previously been used primarily for processing electrical devices on a semiconductor substrate, where the principal concern is the electrical properties of materials, usually characterized by the dielectric constant and breakdown voltage. However, for optical applications refractive index tolerances can be constrained, for example, to within the range of ±0.0002, which is substantially more stringent (e.g., about 2 orders of magnitude more stringent) than for semiconductor electronic fabrication processes.
At present, known integrated optical core manufacturing techniques have a variety of shortcomings. For instance, one prior art technique uses flame hydrolysis for depositing the optical core. However, flame hydrolysis processes are costly and lead to poor uniformity on large substrates. Another prior art technique uses a low pressure chemical vapor deposition process (“LPCVD”). However, known LPCVD techniques suffer from low deposition rates that result in increased production costs. Furthermore, prior art plasma enhanced chemical vapor deposition (“PECVD”) processes suffer from high propagation losses, high stresses, or poor uniformity.
Thus, there is a need for an improved method of manufacturing an integrated optical core to allow for the efficient integration of optical systems on a single structure.